Travis S. Grimes

Optical Spectroscopy Study of Lanthanide Organic Phase Complexes in the TALSPEAK Separations Process

Time-resolved fluorescence spectroscopy and Fourier transform IR spectroscopy have been applied to characterize the coordination environment of lipophilic complexes of Eu(3+) with bis(2-ethylhexyl)phosphoric acid (HDEHP) and (2-ethylhexyl)phosphonic acid mono(2-ethylhexyl) ester (HEH[EHP]) in 1,4-diisopropylbenzene (DIPB). The primary focus is on understanding the role of lactate (HL) in lanthanide partitioning into DIPB solutions of HDEHP or HEH[EHP] as it is employed in the TALSPEAK solvent extraction process for lanthanide separations from trivalent actinides. The broader purpose of this study is to characterize the changes that can occur in the coordination environment of lanthanide ions as metal-ion concentrations increase in nonpolar media. The optical spectroscopy studies reported here complement an earlier investigation of similar solutions using NMR spectroscopy and electrospray ionization mass spectrometry. Emission spectra of Eu(3+) complexes with HDEHP/HEH[EHP] demonstrate that, as long as the Eu(3+) concentration is maintained well below saturation of the organic extractant solution, the Eu(3+) coordination environment remains constant as both [HL](org) and [H(2)O](org) are increased. If the total organic-phase lanthanide concentration is increased (by extraction of moderate amounts of La(3+)), the (5)D(0) → (7)F(1) transition singlet splits into a doublet with a notable increase in the intensity of both (5)D(0) → (7)F(1) and (5)D(0) → (7)F(2) electronic transitions. The increased multiplicity in the emission spectra indicates that Eu(3+) ions are present in multiple coordination environments. The increased emission intensity of the 614 nm band implies an overall reduction in symmetry of the extracted Eu(3+) complex in the presence of macroscopic La(3+). Although [H(2)O](org) increases to above 1 M at high [HL](tot), this water is not associated with the Eu(3+) metal center. IR spectroscopy results confirm a direct Ln(3+)-lactate interaction at high concentrations of lanthanide and lactate in the extractant phase. At low organic-phase lanthanide concentrations, the predominant complex is almost certainly the well-known Ln(DEHP·HDEHP)(3). As lanthanide concentrations in the organic phase increase, mixed-ligand complexes with the general stoichiometry Ln(L)(n)(DEHP)(3-n) or Ln(L)(n)(DEHP·HDEHP)(3-n) become the dominant species.

Lactic Acid Partitioning in TALSPEAK Extraction Systems

The TALSPEAK process, developed in the 1960s at the Oak Ridge National Laboratory and demonstrated in various forms up to pilot scale elsewhere, has emerged as a primary method for partitioning of trivalent transplutonium actinides from fission product lanthanides in advanced nuclear fuel cycles. The baseline process relies on the monoacidic dialkyl phosphoric acid extractant bis-(2-ethylhexyl)phosphoric acid (HDEHP), the aqueous complexant diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA), and high concentrations of a pH 3.5 buffer, usually 1-2 M lactic acid (HL). Previous reports have established that HL not only buffers the pH, but also increases phase transfer kinetics, improves the radiation stability of DTPA, and has been implicated as a possible participant in the formation of ternary complexes in both the aqueous and organic phases. One feature central to the interpretation of these results is the extent of partitioning of HL between the phases. In this study, 14C-labeled HL has been used to examine the details of its partitioning between aqueous solutions and organic solutions relevant to the possible deployment of TALSPEAK. It is found that, contrary to previous reports, HL partitions into the organic phase independently of the amount and identity of the metal ions that are present.